Piezoelectric crystal with moistureproof coatings



Dec.- 21, 1948. A. M. RQBINSON I 2,456,995

PIEZQELECTRIC CRYSTAL WITH MOISTURE PROOF COATINGS Filed April 18, 1946 INVENA'DRZ k /7 IE lg HRTHURN fioa/Nso/v Patented Dec. 21,

PIEZOELECTRIC CRYSTAL WITH MOISTUREPROOF COATINGS Arthur M. Robinson, Boston, Mala, alsignor, by mesne assignments, to The Brush Development Company, Cleveland, Ohio, a corporation 'of Ohio Application April 18, 1948,80 No. 883,289

The present invention relatest'o the protection of the surface of piezo-electric crystals and especially plates and assemblies of plates made from water-soluble piezo-electric crystals so that the surface between the electrodes and between adjacent plates shal remain in a permanently non-conductive condition even in the presence of moisture which would otherwise impair the performance of the crystal. Such plates have heretofore been generally provided'with foil or other electrodes on their two major faces for establishing an electrostatic field in the plate, and

face-to-face assembliesof such plates with elec-.

trodes secured thereto between their adjacent faces are widely used in microphones, phonograph pickups, etc. Such muftiple plate assemblies are described in the reissue patent to Charles B. Sawyer, No. 20,213, dated December 22, 1936, and No. 20,680, dated March 29, 1938, the patent to Alfred L. W. Williams, No. 2,106,143, dated January 18, 1938, and the patent to John H. Ream, No. 2,266,333, dated December 16, 1941. The performance of these devices may be seriously impaired when minute amounts of moisture exist on the edges of the plates or assemblies. The electrical leakage paths between electrodes are short and wide, conditions generally unfavorable for maintenance of high insulation resistance, andin the case of water soluble salt crystals the leakage problem is unusually severe because any moisture present dissolves some of the salt to form a solution of a much higher electrical conductivity than that of water. In extreme cases, sufficient crystalline material may be dissolved, bothon the edges and under the electrodes to destroy permanently the usefulness of the crystal. Piezoelectric crystals of this type usually have a low melting point and often may be damaged when subjected to a temperature substantially lower than their melting temperature.

The present invention relates to moisture proof ing piezo-electric crystals which tend to absorb moisture and cause electrical failure of the crystal. Piezo-electric crystals of this typeusually have a low melting point and often may be damaged when subjected to a temperature substantially lower than their melting temperature. An illustrative example of this type of crystal is a Rochelle salt crystal having a melting point between 70" and 80 C., but which may be damaged and rendered unsuitable as a piezo-electric crystal if subjected to a temperature as great as about 55 C. For various reasons we l known to the art, the film thickness of the protective coating should not exceed 0.005 inch. This makes'it very dimcult s Claims. (Cl. 111-321) 2 to coat the crystal with a material which will prevent the transfer of moisture vapor.-

Prior to the present invention, a certain degree of moisture protection has been achieved in commercial practice by wrapping the crystal in metal foil and then coating with asphalt or similar material. From a production point of view, this method is time-consuming, expensive and requires special skill for the satisfactory application of the metal foil.

In accordance with the present invention, I envelope the crystal with a metal film by electrodeposition. The metal film cannot be deposited directly upon the crystal by electro-deposition because the crystal would be damaged by immersion in the electroplating solution. However, I have discovered that the crystal can be coated first with a thin adherent layer of a non-conductive moisture-resistance organic film forming material, such as shellac, nitrocellulose, ethyl cellulose, etc. This affords the crystal sufficient protection against moisture so that it can be immersed'in an electroplating bath and coated with a metal film by electroplating.

To assist in a further understanding of the invention, a piezo-electric crystal embodying my invention is shown in the accompanying drawings, in which:

Fig. 1 shows two plates with applied electrodes;

Fig. 2 shows the plates superimposed on each other and cemented together;

Fig. 3 shows the assembly after the application of an outer electrode;

Fig, 4 shows the assembly after the application of the non-conductive coating, and

Fig. 5 shows the assembly after metal plating.

Referring to the drawings, at 10 and I2, Fig. 1,

are shown two crystal plates whose edges are preferably slightly rounded. To these are cemented el ectrodes II and il, a conductive lead being secured to one of them at ll. At I! the plates are then cementedtogether as illustrated in Fig. 2, and an outer conductive lead 23 (see Fig. 3) is secured to the plate ii. The crystal is then coated with a solution of the organic filmforming material in a solvent which is not a sol-- vent for the crystal and then the solvent is dried out. The surface of the non-conductive coating indicated at 3 (Fig. 4;) then is sensitized by treatment with a solution of a stannous salt, preferably, in the presence of hydrogen ions. This is followed'by depositing a film of silver upon the sensitized surface by treatment with a solution of a silver salt and a reducing agent. The crystal then is coated with a metal film It, see Fig. 5,

such as copper. by electro-depositlon. It the nature of the electrod ed crystal is such that a potential diiierenceis to be developed or applied between diflerent portions of electrode means ad- Jacent the surface of the crystal, the crystal should be insulated from the copper film to prevent shortcircuiting of the electrode means. This may be done by coating with the non-conductive organic film-forming material. Preferably, the solution of the organic fllm forming material used forforming the inner moisture resistant coating contains a finely divided filler to assure a coating of sufficient thickness at the corners and edges of the crystal. The finely divided tiller may be either electrically conductive or non-conductive depending upon the type of crystal being prepared. Thus, if the piezoelectric crystal being prepared consists of a single crystal between two electrodes or if it consists of a plurality of series connected plates, an electrically non-conductive filler, such as powdered mica. may be used. If the piezo-electric element is of the type known as a parallel assembly. having for example two crystals cemented together with an electrode between them and two external electrodes connected together, an electrically'conductive filler, such as graphite, may be used,

While a resinous coating ailfords the crystal sunlcient temporary protection so that it can be coated with a metal film by electroplating, it is not an adequate protection against moisture since in time it permits the transfer of enough moisture to ruin the crystal.

The practice of the invention is illustrated by the methods for coating Rochelle salt crystals described in the following examples.

EXAMPLE I -A Rochelle salt piezo-electric element consisting of a single slab out along the proper axis and having two electrodes cemented to the flatsurfaces of the slab was dipped twice in theiollowing solution and the solvent dried out. This operation was repeated to provide a coating of adequate thickness around the crystal and the pertions of the electrodes adjacent the surface of the crystal.

Per cent by weight Spar varnish 57.1 Toluol 7.9 Powdered mica (200 mesh) 29.7 Powdered mica (180 mesh) 5.3

Per cent by weight Btannous chloride 25.0 Distilled water 71.0 Concentrated hydrochloric acid 5.0

The sensitized or tinned" crystal then was immersed in distilled water for 5 minutes and then silvered by immersing in the following solution for 5 minutes or forv a time necessary to deposit a suitable him of silver upon the sensitized crys- Savanna; Bows-ion Parts Solution A 1 Solution 3 9 I Solution A Sugar gms.-- 90 Water cc. 250 Nitric acid (concentrated) -..do. 4 Ethyl alcohol dc 175 Water to make do 1000 solution B Silver nitrate gms. 5 Water cc. 400

Ammonium hydroxide (28-29% NHa) until precipitate is nearly redissolved.

Ammonium hydroxide until precipitate is nearly redissolved Then filter After immersion in the silvering solution the crystal was rinsed in distilled water and a layer of copper applied electrolytically. This may be applied directly to the moist crystal but it is preferred to dry the crystal before plating. The following plating solution was used.

Corrra Pts'rmc Barn Copper sulfate gms. 210

Sulfuric acid (concentrated) "cc.-- 26 Water do 1000 Current 10 amperes per square foot at C.

EXAMPLE II A Rochelle salt piezoelectric element similar to the elements described in Sawyer Reissue Patent 20,213 consisting essentially of two slabs cut along a proper axis and cemented together with an electrode between them having a lead extending out from the assembly, and having an external electrode, with a lead extension, covering the outside surface was dipped twice as described in Example I in a mixture of 20 parts by weight of graphite and parts by weight of the following lacquer. This makes the coating electrically conductive so that the external electrode of the crystal may be used as the electrode in the subsequent copper plating.

Lacqpza 56 sec. nitrocellulose (30% alcohol) 50 "Superbeckacide #1003" (pure phenoldehyde resin) 18 .Beckosol #1224 (an alkyd resin solution); '10 Solvent 325 Sonvanzr Ethyl acetate 15 Butyl acetate 20 Amy] acetate 10 Butyl alcohol l0 iCellosolve 10 Toluol 35 The graphite in the coating renders the coatin electrically conductive so that the external electrode of the crystal may be used for connecting the crystal to the source of electric current during the subsequent copper plating operation.

The thus coated crystal was sensitized, silvered and then coated with an electrically deposited film of copper in the manner described in Example 1, except the external electrode of the crystal was connected via its lead extension to the source of electric current during the copper plating operation.

Crystals coated with a metal film as described have withstood immersion in water at 100 F., for over 4 months without fai1lng.- For the purpose of test, when the electrical resistance, measured between electrodes, of the crystal has dropped to 1 megohm the crystal is considered to have failed. Asphalted foil wrapped crystals,

tested in the same manner. failed in from 38 to- 95 hours.

The method of the invention is especially useno for coating plates and multiplate assemblies of piezo-electric crystals of Rochelle salt but it is useful also for coating whole crystals, and sections and assemblies of other than plate shape, and is useful with other piezo-electric crystals, such as ammonium dihydrogen phosphate and lithium sulfate.

I claim: I

1. A piezo-electric salt crystal having a moisture-proof continuous coating comprising a waterproof inner layer and outer layer of metal plating.

2. A piezo-electric salt crystal having a moisture-proot continuous coating comprising an in- 'ill ner layerof a non-conductive, organic film forming material deposited from a non-aqueous solution and an outer layer of metal plating.

3. A piezo-electric Rochelle salt crystal having a moisture-proof coating comprising an inner layer which is essentially a water proof, nonconductive organic film forming material and an which the tiller is graphite.

ARTHUR M. ROBINSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,248,057 Bond July 8, 1941 2,266,333 Ream Dec. 16, 1941 2,386,27 9 Tibbetts' Oct. 9, 1945 

